Process for separating gases at cryogenic temperatures

ABSTRACT

The invention relates to a process employing a multi-directional heat pump and a cryo-trap to separate gases. One embodiment relates to a process that includes heating a gaseous stream comprising carbon dioxide, hydrogen, and a push gas to produce a first intermediate stream comprising an amount of methane; contacting the first intermediate stream with a first trapping material to remove an amount of a component selected from the group consisting of oxygen, water, nitrogen, carbon dioxide, and combinations thereof from the first intermediate stream to produce a second intermediate stream; introducing the second intermediate stream to a separation zone comprising a surface of a second trapping material; employing a multi-directional heat pump to maintain the separation zone at a retaining temperature; retaining methane in the separation zone, while purging hydrogen from the separation zone; employing the multi-directional heat pump to adjust the separation zone to a releasing temperature; and releasing methane from the separation zone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to separating gases, and more specifically to separating gases at cryogenic temperatures.

2. Description of the Related Art

Many automated chemical processes involve the purification of one species through separation from unwanted species by boiling point differences. Such is the case in the production of radiolabeled Methyl Iodide. In the production of ¹¹C labeled Methyl Iodide, hydrogen is first reacted with [¹¹C]CO₂ over a nickel catalyst to produce [¹¹C]CH₄. The [¹¹C]CH₄ is then reacted with gaseous iodine to produce [¹¹C] labeled Methyl Iodide. Hydrogen is a necessary part of the first step of this reaction, but hydrogen is a contaminant in the second step and will both prevent the desired reaction and cause damage to downstream equipment if not removed. Similarly, water, oxygen, and nitrogen are all byproducts and contaminants of the first step of the process and pollutants in the second step. However, unlike hydrogen, these byproducts (water, oxygen, and nitrogen) can be removed with trap materials such as ASCARITE™, a sodium-hydroxide-coated silica and phosphorus pentoxide (P₂O₅). Room temperature trap materials such as these are not useful for separating hydrogen from the product stream in the automated production of [¹¹C] labeled Methyl Iodide.

Cryogenic liquid cooled traps designed to take advantage of the boiling point difference between two species (such as hydrogen and methane, in this specific case) can be designed into automated systems. While simple in design, these traps are not without their disadvantages. First, initial and/or repeated cooling of these traps produces large volumes of effluent gas, which can cause regulatory concerns in some cases. Second, handling systems for cryogenic liquids are bulky, and expensive. Third, although most cryogenic fluids like liquid nitrogen are relatively inexpensive, a constant inventory of cryogenic fluids must be maintained. Fourth, personnel must be trained in the handling of cryogenic fluids. Finally, the trap must constantly be refilled, either manually, which increases radiation risk if there is residual radiation in the system from a prior run, or automatically, necessitating expensive equipment and reducing efficiency. This limitation is a particular concern when dealing with radiolabeled compounds as residual radioactive species around the trap can create significant radiation fields. These fields increase radiation exposure rates when personnel refill the trap and/or can dramatically increase the time between radio syntheses. The amount of time the trap can remain at temperature is limited by the inventory of liquid nitrogen or other coolant. Furthermore, elimination of operator intervention would be particularly desirable, for separating materials that pose a radiation risk to workers.

Thus, there is a need for a new process for separating gases at cryogenic temperatures.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a remote, cryogenic trap coupled to a multi-directional heat pump or cryo-refrigerator for small volumes of gas.

Another embodiment of the present invention relates to a process comprising employing a multi-directional heat pump to maintain a separation zone at a retaining temperature; introducing a gaseous stream comprising a desired component and an undesired component to the separation zone; retaining an amount of the desired component in the separation zone, while purging an amount of the undesired component from the separation zone; employing the multi-directional heat pump to adjust the separation zone to a releasing temperature; and releasing the desired component from the separation zone, wherein the separation zone comprises a surface of a trapping material. Preferably, the desired component is radio-labeled methane (¹¹CH₄). Preferably, externally imposed flow from one or more pressurized push gases is a sufficient mechanism to drive the gaseous stream comprising the desired component and the undesired component into the separation zone, and no fan or impeller is employed. Preferably, a push gas is employed to purge the amount of the undesired component from the separation zone. Preferably, the push gas comprises helium. Preferably, the gaseous stream further comprises hydrogen. Preferably, the gaseous stream further comprises the push gas. Preferably, the retaining temperature is less than or equal to the boiling point of the desired component, and wherein the releasing temperature is greater than or equal to the boiling point of the desired component. Preferably, the retaining temperature is less than or equal to −161.6 degrees Celsius, and the releasing temperature is in the range of from −161.6 degrees Celsius or greater. Particularly preferably, the releasing temperature is in the range of from −161.6 degrees Celsius to −120 degrees Celsius or greater. It is also particularly preferable that the desired component is released from the separation zone in a gaseous state. Preferably, the trapping material comprises an ethylvinyl benzene-divinyl benzene copolymer.

Another embodiment of the present invention relates to a process for producing methane comprising heating a gaseous stream comprising carbon dioxide, hydrogen, and a push gas at a temperature in the range of from 300 to 500 degrees Celsius to produce a first intermediate stream comprising an amount of methane; contacting the first intermediate stream with a first trapping material at a temperature in the range of from 15 to 30 degrees Celsius to remove an amount of a component selected from the group consisting of oxygen, water, nitrogen, carbon dioxide, and combinations thereof from the first intermediate stream to produce a second intermediate stream; introducing the second intermediate stream to a separation zone comprising a surface of a second trapping material; employing a multi-directional heat pump to maintain the separation zone at a retaining temperature; retaining methane in the separation zone, while purging hydrogen from the separation zone; employing the multi-directional heat pump to adjust the separation zone to a releasing temperature; and releasing methane from the separation zone. Preferably, the process employs radiolabeled carbon dioxide (¹¹CO₂) and produces ¹¹C labeled methane. Preferably, a push gas is employed to purge hydrogen from the separation zone. Preferably, the push gas comprises helium. Preferably, the retaining temperature is less than or equal to −161.6 degrees Celsius, and the releasing temperature is −161.6 degrees Celsius or greater. Particularly preferably, the releasing temperature is in the range of from −161.6 degrees Celsius to −120 degrees Celsius. Preferably, the first trapping material is selected from the group consisting of sodium-hydroxide-coated silica, phosphorus pentoxide (P₂O₅) and combinations thereof. Preferably, the second trapping material comprises an ethylvinyl benzene-divinyl benzene copolymer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, and accompanying drawings.

FIG. 1 shows an embodiment of a process employing a multi-directional heat pump/trap combination.

It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention as well as to the examples included therein. All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may include numbers that are rounded to the nearest significant figure.

Embodiments of the invention relate to a remote, cryogenic trap coupled to a multi-directional heat pump. Preferably, the invention makes use of a high-efficiency cryo-refrigerator, and has the advantage of being temperature controlled. Multiple temperature set points are preferably used to selectively trap and/or selectively release different species. Preferably, the trap is coupled to a cryorefrigerator whose temperature can be increased or decreased at will (hereinafter referred to “bidirectionality”). The user is preferably no longer tied to a single temperature, for example the single temperature of a cryo bath (such as the boiling point of nitrogen, 77K). Instead, the user can preferably separate materials by adjusting and stepping the cryo-refrigerator temperature to effect separations of multiple compounds, even where the boiling points might only be separated by a few degrees. The process according to the present invention preferably comprises remote, rechargeable, bidirectional or multi-directional, temperature-controlled separation of small volumes of material using a cryo-refrigerator.

According to various embodiments of the present invention, a high-efficiency multi-directional heat pump or multi-directional cryo-refrigerator (e.g. pulse tube, Stirling Engine, Cryocooler or similar device) is employed to provide lift to a conventional cryogenic gas trap device. Preferably, substrate materials are used in conjunction with the multi-directional heat pump to increase the efficiency of the trap. After the desired gas has been trapped, condensed, and properly purified, the multi-directional heat pump is preferably reversed to heat the trap and release the gas for further processing or capture in an external vessel. Preferably, multiple temperature set points on the multi-directional heat pump are also used to trap and release multiple gases and separate them by boiling point and/or by affinity to the trapping material.

Preferably, operator intervention is not required to cycle the trap from room temperature to cryogenic temperatures multiple times, or to maintain the zone at cryogenic temperatures for extended periods of time. The invention preferably results in zero emissions from evaporation of cryogenic cooling fluids.

An embodiment of a device according to present invention consists of a particle accelerator (101), which produces gaseous ¹¹CO₂. The gaseous ¹¹CO₂ is delivered through transfer lines (102) in the presence of nitrogen, oxygen and helium push gas (103). These gases are combined with hydrogen from supply bottle (104) in the methane oven (105) at 400 degrees Celsius to produce ¹¹CH₄ (methane). At point (106) in the process, ¹¹CH₄ is present with oxygen, hydrogen, water, nitrogen, helium and unreacted ¹¹CO₂. A first process filter (107) preferably comprising ASCARITE™, a sodium-hydroxide-coated silica, and a second process filter (108), preferably comprising phosphorus pentoxide (P₂O₅), remove the oxygen, water, nitrogen and unreacted ¹¹CO₂. The remaining ¹¹CH₄, helium, and hydrogen at point (109) are transferred by input line (110) through insulation (111) to the cryogenic trap (112). The trap (112) is packed with a trapping material (113), preferably PORAPAK Q™, an ethylvinyl benzene-divinyl benzene copolymer, preferably 50-80 mesh, available from Applied Science Laboratories, Inc. Particularly preferably, the trap (112) is cooled to −165 degrees Celsius prior to the start of the chemical synthesis. The ¹¹CH₄ is trapped by the trapping material (113) while the hydrogen and helium push gas pass through trap to valve (114) and out through the waste line (115). After the helium push gas has pushed all the hydrogen out the waste line (115), the heat pump (116) is set to −120 degrees Celsius where the purified and concentrated ¹¹CH₄ gas is routed to valve (114) through line (117) for further processing to ¹¹C labeled Methyl Iodide. First balance weight 119 and second balance weight 120 are connected to the cryocooler through a planar spring 118.

Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.

The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C §112, sixth paragraph. In particular, the use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C §112, sixth paragraph. 

1. A process comprising employing a multi-directional heat pump to maintain a separation zone at a retaining temperature; introducing a gaseous stream comprising a desired component and an undesired component to the separation zone; retaining an amount of the desired component in the separation zone, while purging an amount of the undesired component from the separation zone; employing the multi-directional heat pump to adjust the separation zone to a releasing temperature; and releasing the desired component from the separation zone, wherein the separation zone comprises a surface of a trapping material.
 2. The process of claim 1, wherein the desired component is radio-labeled methane (¹¹CH₄).
 3. The process of claim 1, wherein a push gas is employed to purge the amount of the undesired component from the separation zone.
 4. The process of claim 3, wherein the push gas comprises helium.
 5. The process of claim 1, wherein the gaseous stream further comprises hydrogen.
 6. The process of claim 1, wherein the gaseous stream further comprises the push gas.
 7. The process of claim 1, wherein the retaining temperature is less than or equal to the boiling point of the desired component, and wherein the releasing temperature is greater than or equal to the boiling point of the desired component.
 8. The process of claim 7, wherein the desired component is released from the separation zone in a gaseous state.
 9. The process of claim 2, wherein the retaining temperature is less than or equal to −161.6 degrees Celsius, and wherein the releasing temperature is −161.6 degrees Celsius or greater.
 10. The process of claim 1, wherein the trapping material comprises an ethylvinyl benzene-divinyl benzene copolymer.
 11. A process for producing methane comprising heating a gaseous stream comprising carbon dioxide, hydrogen, and a push gas at a temperature in the range of from 300 to 500 degrees Celsius to produce a first intermediate stream comprising an amount of methane; contacting the first intermediate stream with a first trapping material at a temperature in the range of from 15 to 30 degrees Celsius to remove an amount of a component selected from the group consisting of oxygen, water, nitrogen, carbon dioxide, and combinations thereof from the first intermediate stream to produce a second intermediate stream; introducing the second intermediate stream to a separation zone comprising a surface of a second trapping material; employing a multi-directional heat pump to maintain the separation zone at a retaining temperature; retaining methane in the separation zone, while purging hydrogen from the separation zone; employing the multi-directional heat pump to adjust the separation zone to a releasing temperature; and releasing methane from the separation zone.
 12. The process according to claim 11, wherein the process employs radiolabeled carbon dioxide (¹¹CO₂) and produces ¹¹C labeled methane.
 13. The process of claim 11, wherein a push gas is employed to purge hydrogen from the separation zone.
 14. The process of claim 13, wherein the push gas comprises helium.
 15. The process of claim 11, wherein the retaining temperature is less than or equal to −161.6 degrees Celsius, and wherein the releasing temperature is −161.6 degrees Celsius or greater.
 16. The process of claim 11, wherein the first trapping material is selected from the group consisting of sodium-hydroxide-coated silica, phosphorus pentoxide (P₂O₅) and combinations thereof.
 17. The process of claim 11, wherein the second trapping material comprises an ethylvinyl benzene-divinyl benzene copolymer. 